To answer this question, we need to understand subject-verb agreement.
The subject of the sentence is "The large companies," which is plural. When the subject is plural, the corresponding verb should also be plural.
Option A) "need" - This option is correct because it agrees with the plural subject "companies." Therefore, it should be "The large companies need a cheap source of labor."
Option B) "needs" - This option is incorrect because it is singular and does not agree with the plural subject "companies."
Option C) "None of the above" - This option is incorrect because option A is correct.
Option D) "Both a and b" - This option is incorrect because only option A is correct.
The correct answer is A) "need." This option is correct because it agrees with the plural subject "companies." Therefore, the correct sentence is "The large companies need a cheap source of labor."